The present invention relates to an image processing technique, and more particularly to a method and apparatus for applying the pixel-density conversion to a continuous-tone image binarized by the dither method.
The pixel-density conversion process of an original image has been used in various types of image processings. A typical application of this process is the pixel-density conversion of the image in transferring the image data between different types of facsimiles with different scanning line densities. Another application of the process is found in the computer-assisted office equipment such as a work station for generating documents containing continuous-tone images such as photographs, in addition to characters and figures. In such equipment, the pixel-density conversion process is used for changing (enlargement or reduction) the size of the continuous-tone image.
The pixel-density conversion methods for the original image, which have widely been used, are the SPC (selective processing conversion) method, the logical summing method, and the projecting method. These kinds of methods are effective when they are applied for the binary image having picture elements (pixels) with only one of two-levels, i.e., "white" and "black", such as the images of the document containing only characters, for example. These methods, however, are ineffective for the enlargement/reduction of continuous-tone images such as photographs. Specifically, if those methods are applied for changing the size of such images, the halftone, or "gray" level of the "white" and "black" image can inappropriately be depicted on the screen. The image tone characteristic of the converted image is degraded, resulting in great deterioration of the picture quality of the converted image.
In this field of technique, there has been known the use of the dither method for the purpose of converting a pixel density in the continuous-tone image. The dither method may be defined as a technique for representing the entire gray scale of a picture image, in which a multilevel input image signal is compared with a position-dependent set of thresholds, and pixels are set to "white" only where the image input signal exceeds the threshold. If the dither method is used, the pixel-density conversion for the continuous-tone image, such as a photograph image of the facsimile communication, whose commercial market is expanding year by year, or photographs contained in the office document, can be performed, while keeping a good picture quality, which is superior to that produced by the prior pixel-density conversion method.
At present, however, the picture quality of the image converted by the dither method (this image will be referred to as a "converted image") is still unsatisfactory. In the prior pixel-density conversion by the dither method, a multilevel input image is subjected to the primary dither process, to generate a binarized, or two-level original image signal. If this image is simply reduced at a selected conversion ratio by using an appropriate known method, such as the SPC method, the coarsened and irregular image areas in tone, and a Moire pattern appear in the reproduced image. The result is an unsatisfactory tone characteristic of the reproduced image. The prior technique was almost unable to solve this problem. This fact provided a serious problem in the image processing system handling the continuous-tone images.